AbilityClick

If you try using an ability just before GCD ends, the new ability is queued. It is executed as soon as the GCD ends. If you are running a tight rotation, you want to queue abilities as soon as possible, which leads to button mashing.

This addon plays a click every time an ability is queued to let you know that you can stop mashing that key. It works best if your rotation doesn't involve repeating the same ability several times in a row (example: protection paladin rotation).

I have noticed a lot of issues with the GCD since they implemented the change. A lot of what appears to be double GCDs or partial second GCDs occurring. This might be why the below guy doesn't like the queuing. Or it could be that there doesn't seem to be a reliable way to queue a different ability if you needed to. Although in the long run, it is an improvement.